[Python] Fix String.Concat spread operator issue in Python transpilation

[dbrattli]

Summary

• Fixed incorrect hasSpread flag usage for String.Concat in Python Replacements
• Simplified Rust concat implementation to accept varargs of strings directly
• Updated type stub to match the new signature
• Reverted linting changes to pyproject.toml that got removed by mistake

Problem

When transpiling F# code like:

let cssNew = $$""".{{classAttr}}:hover {background-color: #eee;}"""

The generated Python code was:

concat(".", "item-panel", *":hover {background-color: #eee;}")

The * spread operator was incorrectly applied to the last string argument, which would spread the string into individual characters. This happened to work because strings are iterable in Python and the characters would be joined back together, but it was inefficient and semantically incorrect.

Solution

• Removed hasSpread = true from the String.Concat case in Python Replacements - the arguments should be passed directly without spreading
• Simplified the Rust concat function from a complex implementation handling iterables to a simple varargs join
• Updated the .pyi type stub to reflect the correct signature

Generated code after fix

concat(".", "item-panel", ":hover {background-color: #eee;}")